I've been a live-in caregiver for my dad for almost 2 years. He has a significant hearing loss, even with a hearing aid. He gets angry if asked to turn it down. Even with earplugs, which sometimes cause ear infections and raw ears, it is loud. I've always had good hearing, but now I'm beginning to have a hard time hearing people.
I've heard of TV "ears" that apparently provide sound directly through the headset so the tv doesn't have to be turned up as loud. I've never used them so I can't report on how well they work.
Something you could do is kind of throw his behavior right back at him. Wear ear protectors that are worn for construction work, shooting/target practice, etc. I like the big ear muffs; I wear them when I mow the lawn, even when I use a drill, vacuum or use a mix master.
Is there any way you can get your dad to do something besides watch TV for 12 hours? I'm guessing that at some point he's going to have some mobility problems. Is he interested in any particular hobby? Do you think you could get him to go to a senior center for entertainment?